The invention relates to a planar optical isolator having a magnetooptical gyrotropic layer and an anisotropic birefringent layer which are proportioned so that the non-reciprocal rotation of polarisation of the light (Faraday effect) resulting from the action of a magnetic field in the forward direction of the isolator is at least substantially negatively equal to the reciprocal rotation of polarisation as a result of the birefringence, so that a TE-mode in the forward direction is guided with low damping, measures being taken for the intensive attenuation of at least one TM-mode.
In this case it relates to isolators in which the functions of a non-reciprocal means for rotating the polarisation, a reciprocal means for rotating the polarisation and of polarisers are combined in one single component. The plane of polarisation of the light (TE-mode) is not varied in the forward direction. Compact and comparatively short (approximately 10 mm) components are obtained.
Gyrotropic, optically low-damping and magnetooptical YIG crystal layers of a stripe waveguide permit a comparatively strong non-reciprocal rotation of polarisation. In general there is referred to YIG layers also when yttrium was substituted entirely or partly by other rare earth metal elements.
The reciprocal rotation of polarisation which occurs with the same sense of rotation in the forward direction and in the reverse direction of the polariser is obtained by anisotropic birefringent properties of waveguide layers.
For a pre-determined wavelength of the light the stripe waveguides can be proportioned in their magneto optical and birefringent properties so that in the forward direction the non-reciprocal and the reciprocal rotation of polarisation are negatively the same. For example, a TE-mode oscillating perpendicularly to the plane of the layer can be guided through the insulator without rotation of polarisation. In the reverse direction on the contrary, the said TE-mode is increasingly converted into a TM-mode oscillating perpendicularly thereto, since in the said direction the reciprocal and the non-reciprocal phase rotations have a supporting effect in the same sense.
In the isolator known from IEEE Journal of Quantum Electronics 1982, pp 1975 to 1981, in which an isotropic magnetooptical waveguide layer is provided between cladding layers of anisotropic substrate and an anisotropic birefringent crystal layer, metal layers are provided at the ends of the isolator which exert a polarising effect in that they only damp the TM-modes selectively. The building length of the said known isolator must now be matched in such a manner that a complete conversion, for example, of a TE-mode into a TM-mode must have occurred on the return path. This is achieved by superposition of a Faraday rotation through 45.degree. and a rotation of 45.degree. caused by the birefringence. The TM-mode is finally damped by the metal strips, so that in the final result substantially no light emanates from the isolator in the reverse direction. Besides an accurate phase matching (.DELTA..beta.=0) of the TM-and TE-modes to ensure equal velocities of propagation, a careful length matching is required for a useful isolator action of the said so-called 45.degree.-waveguide isolator. In addition, intimate connection of strongly birefringent layer materials with a magnetooptical layer will present difficulties. This latter difficulty also exists in an isolator known from J. Appl. Phys. 1981, pp 3190 to 3199, in which an LiNbO.sub.3 layer is to be connected to a YIG layer. The isolating activity of this known isolator is obtained by conversion of a reverse TE-mode into a radiating TM-mode. Experimentally, isolating activities of only 10 dB were achieved because, probably in spite of a selenium intermediate layer, a sufficiently intimate optical contact of the YIG layer with the LiNbO.sub.3 layer was not obtained and because on the other hand the theoretically required radiation of TM-modes in itself is completely possible only in an infinitely thick LiNbO.sub.3 layer.